Mark Zuckerberg, chief executive of Facebook, in September. On Thursday, he said in a 5,800-word letter that “progress now requires humanity coming together not just as cities or nations, but also as a global community.”CreditCreditJustin Sullivan/Getty Images

In a 5,800-word letter he posted publicly, Mr. Zuckerberg expressed alarm that what was once considered normal — seeking global connection — was now seen by people and governments around the world as something undesirable.

He pledged that he would push Facebook, which has more than 1.8 billion users worldwide, in a direction that would help convince individuals and governments that “progress now requires humanity coming together not just as cities or nations, but also as a global community.”

The letter comes close to a political statement by a chief executive who, as the leader of a global company, is essentially arguing against a tide of isolationism that is rising across the world.

Mr. Zuckerberg, 32, chose to make the statement as an update to his original founder’s letter, which was published in 2012 when Facebook went public. In that letter, he wrote that the social network “was built to accomplish a social mission — to make the world more open and connected.”

In an interview this week at the company’s headquarters in Menlo Park, Calif., Mr. Zuckerberg said that when he started Facebook, “the idea of connecting the world was not controversial.”

“The default assumption was that the world was just incrementally moving in that direction,” he said. “Now, that’s actually a real question.”

Mr. Zuckerberg released his missive amid a fierce debate over the merits of globalization. In the United States, President Trump has displayed a deep streak of nationalism. Last year, Britain voted to quit the European Union. Those moves and others have been taken as signs of how globalization has caused strains — making it more difficult for companies to navigate a new world order.

Against that backdrop, the timing of the letter is notable. While other technology executives, including Jeff Bezos of Amazon and Larry Page and Sergey Brin of Google, regularly update their founder letters each year — often tied to financial results — it is the first time Mr. Zuckerberg has refreshed Facebook’s mission statement since the company’s initial public offering.

Mr. Zuckerberg said his reasons for writing the updated letter began to take shape before last year’s presidential election, spurred by broader trends. He said he recognized that more people were feeling left behind by globalization, and by societal and technological changes. As a result, he wanted to focus different parts of Facebook in a way that helped people better come together.

“We have to build a global community that works for everyone,” he said. “I really don’t have much doubt that this is the right direction to go in the long term.”

Mr. Zuckerberg said he planned to reorient Facebook, which turned 13 this month, around these new realities. His letter is filled with abstract ideas, including the notion of “social infrastructure” and how to create stronger online communities, with few detailed steps about how to realize those goals.

One top priority is building inclusive online communities that are supportive, safe and informed environments, to help strengthen bonds in the offline world as well. Mr. Zuckerberg also stressed the idea of using Facebook to create this “social infrastructure” — essentially a digital means of connecting people around shared interests — mirroring social groups in the physical world like churches, governments and unions. That may encourage connections among people that transcend national and international barriers, he said.

While Mr. Zuckerberg mentioned several concrete steps that Facebook has taken toward some of these goals, he gave few other specifics. He did not address how his vision might conflict with those of political leaders around the world, nor did he mention how emphasizing global connections benefited Facebook’s finances.

Getting to this point did not come easily. Since going public, Facebook has been on a growth tear, becoming one of the world’s biggest and most valuable public entities.

Yet the power that Facebook has amassed has raised hackles globally. The social network is blocked in China, has stumbled in India and is facing a thicket of regulatory questions in Europe. More recently, Mr. Trump’s election prompted an outcry over whether Facebook influenced the American electorate with false stories on its site. The company has scrambled to contain the fallout, undertaking several experiments to better sort through what people see on its site.

A key moment for Mr. Zuckerberg’s shift in thinking about how to be a global company happened six months ago. In September, Facebook censored an iconic photograph that featured a naked 9-year-old girl fleeing napalm bombs during the Vietnam War. Mr. Zuckerberg said the image, titled “The Terror of War,” was rightly censored by Facebook at the time because the company’s content policies did not allow child nudity.

Facebook users disagreed, arguing that the photograph was not titillating but rather illustrated the perils of modern warfare. The social network eventually allowed the image to be posted on its site, noting that it had inherent news value.

Mr. Zuckerberg said the episode made him realize how ineffective Facebook’s content policy was at a global scale, given that cultural norms vary greatly by country. He said Facebook needed to get to a place in which users could perhaps choose their own content policies based on local laws and preferences, a marked departure from the company’s blanket global approach of the past decade or more.

“I don’t think that we, sitting here in California, are best positioned to know what the norms in communities around the world should be,” Mr. Zuckerberg said. “At some point, you just need a more dynamic system where people can just express that themselves.”

Mr. Zuckerberg spent much of his letter focused on the importance of personal relationships. He pointed to “very meaningful groups” — Facebook groups that people frequently engage in and return to — that are often centered on topics like parenting, sports or other shared interests. When people find an interest they are passionate about and form such a group, they are some of the most tightly connected relationships on Facebook, he said.

To foster more of these groups in a safe way, Mr. Zuckerberg said Facebook needed to change some of the ways that it operated. That includes improving community safety with product updates similar to Facebook’s “safety check,” a tool that lets people mark themselves safe during catastrophes. Mr. Zuckerberg said that governments regularly called Facebook to confirm that Safety Check was activated during crises like the massacre at a nightclub in Florida last year.

Mr. Zuckerberg also emphasized Facebook’s role in keeping communities well-informed, which will necessitate tackling misinformation and highly polarized news. He alluded to Facebook’s shifting role as a distributor of news, saying the social network is “not just technology or media.”

Facebook’s goal, Mr. Zuckerberg said, was not to usurp traditional institutions like governments, religious groups and other communities that share interests. (When asked if he wanted to run for president of the United States, he laughed and declined.) Instead, he said, creating tightly knit online groups would make these traditional institutions stronger.

It also would not hurt that such groups and connections could well make Facebook more essential to people, which in turn may increase the company’s profits.

“There’s a social infrastructure that needs to get built to deal with modern problems in order for humanity to get to the next level,” Mr. Zuckerberg said. “I just think it would be good if more people thought about things like this.”

Correction:

An article on Friday about an update to Facebook’s mission statement misstated, in some copies, the location of a nightclub massacre last year. It was in Florida, not Paris. (A nightclub massacre in Paris occurred in 2015.)

A version of this article appears in print on , on Page B5 of the New York edition with the headline: Zuckerberg, Bucking Tide, Takes a Public Stand Against Isolationism. Order Reprints | Today’s Paper | Subscribe